1. Field of the Invention
The present invention relates to a method for continuously electrolytic coloring of strips or wires of aluminum or aluminum base alloys (hereinafter, for brevity, both aluminum and aluminum alloys will be designated "aluminum" in this specification). More particularly, the invention relates to a method for continuously conducting a first anodic oxidation of aluminum strips or wires and then an electrolytic coloration of the anodic coating thus obtained.
2. Description of the Prior Art
For continuously coloring the anodic coating of aluminum strips or wires, a method has hitherto been employed in which an aluminum strip or wire is subjected to a degreasing treatment as a pre-treatment, and after forming an oxide film or layer on the aluminum strip or wire by an anodization, the anodized article is continuously immersed in a dyeing bath containing an organic dye. This method is useful in that aluminum strips or wires of various desired colors can be obtained in a comparatively short period of time, but has the defects that the colored aluminum articles obtained by the method are poor in weatherability and are faded by exposure for a long period of time. Therefore, such a conventional method is unsuitable for building materials, etc., which have recently become in great demand.
On the other hand, as a method of obtaining a colored anodic coating on an aluminum surface having a high resistance in weathering, a method in which aluminum articles are anodized in an electrolytic bath containing an organic acid such as sulfosalicylic acid and a method in which articles of an aluminum alloy containing chromium and manganese are subjected to an anodization treatment in an aqueous sulfuric acid solution are known. In these known methods, the formation of the anodic oxide film or layer and the coloring of the oxide film or layer are conducted simultaneously in the same bath, but there is a difficulty that insufficient coloring is achieved if the thickness of the oxide film or layer formed on the aluminum article or aluminum alloy article is less than about 15 microns although the extent of coloring depends on the electrolytic conditions and the nature of the aluminum alloy. Therefore, in order to obtain desirable coloring using such known methods, a large amount of electrical energy is required, and further, since the oxide film formed by such methods has a high hardness, the oxide film formed on an aluminum strip or wire tends to be cracked when the aluminum article is continuously withdrawn from the anodic oxidation bath, which makes these prior art methods unsuitable for continuous treatment.
We have previously discovered an improved method of coloring aluminum articles by anodizing the aluminum articles in an anodic oxidation bath to form an anodic coating on aluminum, and then electrolytically coloring the oxide coating in an electrolyte containing a specific acid or water-soluble metal salt as disclosed in DT-OLS 2112927. By our previously discovered method, a colored oxide film or layer having high weatherability or fade resistance can be formed on an aluminum or aluminum alloy article such as an aluminum plate without requiring a large amount of electrical energy.
As the results of further investigations, the inventors have found that the method previously discovered can be also applied to the continuous operation of electrolytic coloring of aluminum strips or wires and have discovered a novel and simple apparatus of the present invention suitable for the continuous formation of colored anodic oxide coatings having excellent weatherability or fade resistance on an aluminum strip or wire.